The present invention relates to an axle beam in a wheel suspension system for an industrial vehicle and a method of manufacturing the same.
Conventionally, a rear axle beam for supporting rear wheels in an industrial vehicle such as forklift truck is pivotally connected to a vehicle frame for supporting a rear wheel for the sake of traveling stability of the vehicle and comfort of riding. For example, as disclosed in FIG. 8 of Unexamined Japanese Patent Publication No. 2000-16107, a rear axle beam (31) includes an axle beam body (34) having a first plate 32 and a pair of second plates (33a, 33b) and also includes two pairs of bosses (35a, 35b) for supporting kingpins (not shown). Each pair of the bosses (35a, 35b) is welded on each side of the axle beam body (34) in such a manner that the bosses (35a, 35b) are vertically arranged on the upper side and the lower side, respectively. The rear axle beam (31) is pivotally supported with respect to a rear frame of a vehicle body (not shown) through a pair of center pins (36) which is formed at the middle portion of the axle beam body (34) as seen in the direction of the vehicle width.
Referring to FIG. 8 of Unexamined Japanese Patent Publication No. 2000-16107, one of the center pins (36) is fixedly connected to the first plate (32) that interconnects a pair of the second plates (33a, 33b). Though it is not directly apparent from the above FIG. 8, in view of the arrangement that the center pins (36) are fixedly connected on both front and rear sides of the rear axle beam (31), it is assumed that the paired second plates (33a, 33b) are also interconnected by a plate member 40 on the opposite side relative to the side interconnected by the first plate (32), as shown in FIG. 9A. Then, the other center pin (36) is presumably fixedly connected to the plate member 40. It is noted that FIG. 9A is a perspective view as seen from the opposite side as shown in FIG. 8 of Unexamined Japanese Patent Publication No. 2000-16107. The bosses (35a, 35b) are not shown in FIG. 9A.
The above four plates, that is, the first plate (32), the second plates (33a, 33b) and the plate member 40, cooperate to form a space R surrounded by the front and rear and the top and bottom walls, as shown in FIG. 9B. Thus, the axle beam body (34) forms a tube having a hollow space inside. The shape of the axle beam body (34) increases the rigidity of the axle beam body (34) and ensures the strength thereof.
In the structure disclosed in Unexamined Japanese Patent Publication No. 2000-16107, however, the number of manufacturing processes undesirably increases due to the multiple number (four) of plates to be welded. As indicated by the filled triangles in FIG. 9B, there are as many as four welded portions and the strength tends to be weakened by such relatively large number (four parts) of welded portions. Additionally, it is difficult to ensure stability of strength for an axle beam having many welded portions. To compensate for the insufficient strength due to of the welded portions and to achieve the required strength, the four plates themselves need be made thicker, which only causes increased manufacturing cost.
The four plates prepared, for example, by means of gas fusing may lead to undesired dimensional accuracy and, therefore, the yield rate of material may be reduced to 60% to 70%, thereby increasing manufacturing cost. Additionally, the quality of the welded portions of the bosses (35a, 35b) fixed to the axle beam body (34) by welding tends to be lowered due to poor dimensional accuracy of each component plate of the axle beam body.
Moreover, according to the structure disclosed in Unexamined Japanese Patent Publication No. 2000-16107, since the four plates are used to form the axle beam body (34), the number of components increases, thereby increasing the trouble in controlling such components. Therefore, there is a need for providing an axle beam for an industrial vehicle which reduces the number of components and also reduces the number of manufacturing processes by reducing welded portions.